In recent years, a large number of semiconductor devices are mounted in automobiles. A high breakdown capability (high surge capability) with respect to various kinds of surge voltage, such as ESD (electro-static discharge) is required of these semiconductor devices. Because of this, a surge protection diode (hereafter called a protection diode) is connected in order to protect a power semiconductor element configuring a semiconductor device from surge voltage.
When an external surge voltage or noise voltage, a surge voltage generated by an operation of the power semiconductor element itself, or the like, is applied to the power semiconductor element, a high breakdown capability of the semiconductor device is realized by clamping excessive voltage with the protection diode so that the excessive voltage is not applied to the power semiconductor element.
FIG. 18 is a sectional view showing main portions of a semiconductor device having a heretofore known protection diode. As shown in FIG. 18, in a heretofore known semiconductor device 800, an n-semiconductor layer (hereafter referred to as a high concentration n-semiconductor layer) 2, with an impurity concentration higher than that of a p-semiconductor substrate 1, is disposed on the p-semiconductor substrate 1. An n-semiconductor layer (hereafter referred to as a low concentration n-semiconductor layer) 3, with an impurity concentration lower than that of the high concentration n-semiconductor layer 2, is disposed on the high concentration n-semiconductor layer 2. A p-well layer 34 is disposed in a region (hereafter referred to as a control circuit region) in which is formed a control circuit 91 in a surface layer of the low concentration n-semiconductor layer 3, and a p-well layer 4 is disposed in a region (hereafter referred to as an IGBT region) in which is formed an IGBT 92.
The control circuit region is disposed in a central portion of the low concentration n-semiconductor layer 3. The plural p-well layer 34 are disposed distanced from each other. A MOSFET, diode, resistor Rg (not shown), or the like, configuring the control circuit 91 is disposed in a surface layer of each p-well layer 34. The MOSFET and diode shown in FIG. 18 are each disposed in a different p-well layer 34. Specifically, an n-source layer 35a and an n-drain layer 35b are provided as a MOSFET in the surface layer of one p-well layer 34, while an n-cathode layer 35c is provided as a diode in the surface layer of another p-well layer 34.
A gate electrode 38 is disposed across a gate insulating film 37 on the p-well layer 34 sandwiched by the n-source layer 35a and n-drain layer 35b. A source electrode 54 is in contact with the n-source layer 35a. A drain electrode 55 is in contact with the n-drain layer 35b. Although not shown in the drawing, a MOSFET body diode (parasitic diode) is formed, connected to the source electrode 54, in the p-well layer 34 in which the MOSFET is provided. A cathode electrode 56 is in contact with the n-cathode layer 35c. An anode electrode 57 is in contact with the p-well layer 34 in which the n-cathode layer 35c is provided.
The IGBT region neighbors the control circuit region (on the right side in the plane of FIG. 18). An n-emitter layer 5 of the IGBT 92, which is a power semiconductor element, is disposed in a surface layer of the p-well layer 4. A gate electrode 10 is disposed across a gate insulating film 7 on the p-well layer 4 sandwiched by the n-emitter layer 5 and low concentration n-semiconductor layer 3. An emitter electrode 12 is in contact with the n-emitter layer 5 and the p-well layer 4. Also, a collector electrode 11 configuring the IGBT 92 is disposed on the rear surface of the p-semiconductor substrate 1, which forms a p-collector layer.
Furthermore, a region in which a protection diode 81 (hereafter referred to as a protection diode region) is formed is provided in the low concentration n-semiconductor layer 3 in a region neighboring the control circuit region. The protection diode region neighbors the control circuit region on the side opposite to the IGBT region (on the left side in the plane of FIG. 18), thereby sandwiching the control circuit region. In the protection diode region, an insulating film (LOCOS oxide film) is provided on the low concentration n-semiconductor layer 3.
The protection diode 81 configured of three single unidirectional diodes 81a—a first diode, a second diode, and a third diode—formed of a p-anode layer 21 and an n-cathode layer 22 is disposed on the insulating film 60. For example, the first diode is disposed in the region the farthest from the control circuit region (on the left side in the plane of FIG. 18), while the third diode is disposed in the region nearest to the control circuit region (on the right side in the plane of FIG. 18).
The protection diode 81 is formed of a multi-crystal silicon layer (polysilicon layer). A cathode electrode 51 of the protection diode 81 is in contact with the n-cathode layer 22 of the first diode. The cathode electrode 51 is connected to a gate terminal G of the semiconductor 800. The cathode electrode 51 is connected via the control circuit 91 to the gate electrode 10 of the IGBT 92. An anode electrode 52 is in contact with the p-anode layer 21 of the third diode. The anode electrode 52 is connected to the emitter electrode 12 of the IGBT 92.
That is, the protection diode 81 is inserted between the gate terminal G and the emitter electrode 12 of the IGBT 92. This is equivalent to inserting the protection diode 81 between the gate electrode 10 and emitter electrode 12 of the IGBT 92. Also, as the control circuit 91 is also connected to the protection diode 81, the control circuit 91 is protected from surge.
FIG. 19 is a plan view showing main portions of the protection diode of FIG. 18. The n-cathode layer 22 of the first diode (on the left side in the plane of FIG. 19) is connected via the cathode electrode 51 and a pad electrode to the gate terminal G. The p-anode layer 21 of the first diode is connected to the n-cathode layer 22 of the second diode (in the center in the plane of FIG. 19). The p-anode layer 21 of the second diode is connected to the n-cathode layer 22 of the third diode (on the right side in the plane of FIG. 19). The p-anode layer 21 of the third diode is connected via the anode electrode 52 to the emitter electrode 12 of the IGBT 92. The emitter electrode 12 is connected via the pad electrode to an emitter terminal E.
FIG. 20 is an equivalent circuit showing the semiconductor device of FIG. 18. The three unidirectional diodes 81a configuring the protection diode 81 are connected in series. Of the three unidirectional diodes 81a, the cathode of an upper stage unidirectional diode 81a is connected to the gate terminal G connected to the control circuit 91, while the anode of a lower stage unidirectional diode 81a is connected to the emitter of the IGBT 92. That is, the upper stage unidirectional diode 81a is the first diode of the protection diode 81, while the lower stage unidirectional diode 81a is the third diode of the protection diode 81.
As shown in FIGS. 18 to 20, in the semiconductor device 800, the protection diode 81 formed of a polysilicon layer is formed across the insulating film 60 (LOCOS oxide film) on a surface of the semiconductor (low concentration n-semiconductor layer 3), and the n-cathode layer 22 of the protection diode 81 is connected via the control circuit 91 to the gate electrode 10 of the IGBT 92. Then, the p-anode 21 of the protection diode 81 is connected via the emitter electrode 12 of the IGBT 92 to the n-emitter layer 5. When a surge voltage is applied to the gate terminal G, the protection diode 81 breaks down, clamping the surge voltage, and no high voltage is applied to the gate electrode 10 of the IGBT 92. As a result of this, the IGBT 92 is protected from surge voltage.
Also, in the semiconductor device 800, a breakdown voltage necessary as the protection diode 81 is obtained by the three unidirectional diodes 81a being connected in series. For example, although a gate input voltage is 5V at a time of normal operation in a vehicle mounted application, it may happen that a 12V battery voltage is mistakenly applied to the gate terminal when handling. In order to protect the protection diode 81 itself from this kind of mistaken input, a breakdown voltage equal to or greater than the battery voltage is necessary. Additionally, in order to protect the gate of the IGBT 92, it is necessary that a voltage applied to the gate of the IGBT 92 is equal to or less than the gate breakdown voltage. It is possible to adjust the breakdown voltage of the protection diode 81 by changing the number of the unidirectional diodes 81a. However, when the number of the unidirectional diodes 81a increases, the area of the region (protection diode region) in which the protection diode 81 is formed increases. That is, the chip area of the semiconductor device 800 increases.
Therefore, a description will be given of a method of reducing the area of the protection diode 81. FIG. 21 is a plan view showing main portions of another example of a heretofore known protection diode. FIG. 21 shows a protection diode 82, which realizes a smaller area in comparison with FIGS. 18 and 19. The protection diode 82 is configured of multi-stage unidirectional diodes 81a formed of a polysilicon layer in which plural p-anode layers 21 and n-cathode layers 22 are alternately disposed. Neighboring unidirectional diodes 81a are connected. That is, the protection diode 82 configured of the multi-stage unidirectional diodes 81a is of a configuration wherein the p-anode layers 21 and n-cathode layers 22 of the unidirectional diodes 81a are alternately disposed, neighboring p-anode layers 21 and n-cathode layers 22 are connected, and plural bidirectional diodes having breakdown voltage in two directions are connected in series.
FIG. 22 is a wiring diagram showing a connection relationship between the heretofore known protection diode and a control circuit. The protection diode 82 is inserted between the gate terminal G and the emitter electrode 12 (emitter terminal E) of the IGBT 92, and no current arises until the bidirectional diodes break down in both the forward and reverse directions.
Next, a description will be given of an example wherein a protection diode 83 is inserted between the collector and gate of the IGBT, protecting the semiconductor device from surge voltage. FIG. 23 is an explanatory diagram showing main portions of another example of a semiconductor device having a heretofore known protection diode. FIG. 23 is an explanatory diagram showing a configuration of a semiconductor device in which is mounted the protection diode 83, which is formed of bidirectional diodes. FIG. 23(a) is a plan layout diagram of the whole of the semiconductor device shown in FIG. 23. FIG. 23(b) is a sectional view showing main portions of the semiconductor device shown in FIG. 23. FIG. 23(c) is a plan view showing main portions of the protection diode 83. FIG. 24 is an equivalent circuit diagram showing the semiconductor device of FIG. 23. As shown in FIG. 23, in a semiconductor device 900, the cathode electrode 51 of the protection diode 83 is connected to a collector terminal C via a stopper layer 85 of the IGBT 92, the p-semiconductor substrate 1 that forms a collector layer, and the collector electrode 11. The anode electrode 52 of the protection diode 83 is connected to the gate electrode 10 of the IGBT 92. The stopper layer 85 has the same potential as the collector layer.
As shown in FIG. 23(b), in the semiconductor device 900, for example, an IGBT region is provided between a control circuit region and a protection diode region. The configurations of the control circuit 91 and IGBT 92 are the same as in FIG. 18.
When a high voltage, such as an external surge voltage or a turn-off voltage when the IGBT 92 is switching, is applied to the collector terminal C, the protection diode 83 breaks down before the IGBT 92. Because of the breakdown, a current flows from the collector of the IGBT 92 via the protection diode 83 to the resistor Rg (refer to FIG. 22) in the control circuit 91, and a voltage is generated in the resistor Rg. The voltage is applied to the gate of the IGBT 92, the gate potential of the IGBT 92 rises, and when the gate potential rises to or above the gate threshold value, the IGBT 92 carries out a turning-on operation. As a result of the turning-on operation, no high voltage of a certain value or more is applied to the collector of the IGBT 92.
Also, the protection diode 83, in the same way as the protection diode 82 shown in FIG. 21, is configured of bidirectional diodes formed of a polysilicon layer formed on the insulating film 60. As the p-anode layers 21 and n-cathode layers 23 of the bidirectional diodes are all of a high impurity concentration, the bidirectional diodes are bidirectional Zener diodes. The protection diode 83 is formed by, for example, vapor phase diffusion.
Also, in, for example, Patent Document 1 below, there is proposed a semiconductor device wherein a protection diode is formed of bidirectional Zener diodes. In Patent Document 1 below, the bidirectional Zener diodes are configured of high impurity concentration p-anode layers and n-cathode layers with an impurity concentration lower than that of the p-anode layers.